Cassegrain antenna with absorber to reduce back radiation

ABSTRACT

An antenna using a primary horn radiator and at least one reflector, in which a wave absorber is positioned at a limited area of the center portion of the reflector, for absorbing electric waves which would otherwise reflect from that limited area and be reapplied to the horn radiator, so that reaction between the feed radiator and the reflector is substantially eliminated.

United States Patent Yokoiet al. A

1 51 Oct. 3, 1972 [54] CASSEGRAIN ANTENNA WITH ABSORBER TO REDUCE BACK RADIATION [72] Inventors: Hiroshi Yokoi; Hiroshi Fukumuro,

both of Tokyo, Japan [73] Assignee: Kokusai Denshin Denwa Kabushiki Kaisha, Tokyo-to, Japan 22 Filed: Sept. 15,1970 [2]] Appl.No.: 72,336

[30] Foreign Application Priority Data Primary Examiner-Eli Lieberman Attorney-Robert E. Burns and Emmanuel J. Lobato [5 7] ABSTRACT An antenna using a primary horn radiator and at least Sept. 16, 1969 Japan ..44/727l0 one reflector, in which a wave absorber is positioned at a limited area of the center portion of the reflector, [52] US. Cl ..343/782, 343/837 for absorbing electric waves which would otherwise [51] Int. Cl. ..H0lq 19/14 reflect from that limited area and be reapplied to the [58] Field of Search ..343/78l, 782, 840, 837 horn radiator, so that reaction between the feed radiator and the reflector is substantially eliminated.

1 Claim, 7 Drawing Figures PATENTED IBH 1 1 3,696 436 saw 3 or 3 PRIOR ART CASSEGRAIN ANTENNA WITH ABSORBER TO REDUCE BACK RADIATION This invention relates to improvements of an antenna provided with at least one reflector, such as a paraboloid antenna and a Cassegrain antenna, used in a range of microwave or millimeter wave.

In conventional antennas of this type, an impedance matching characteristic is usually affected by reaction of a reflector, and the elimination of this reaction is very difficult so that it is very difficult to provide an antenna of this type suitable to a wide frequency range.

An object of this invention is to provide an antenna using a reflector which eliminates the above-mentioned reaction by a reflector, and which is capable of improving effectively the impedance matching characteristic of the antenna.

The principle of this invention will be understood from the following detailed discussion taken in conjunction with the accompanying drawings, in which the same or equivalent parts are designated by the same reference numerals, characters and symbols, and in which:

FIG. 1 is a schematic side view explanatory of construction and operation of a conventional paraboloid antenna;

FIG. 2 is a schematic side view illustrating an installed position of a matching plate used in a conventional paraboloid antenna;

FIG. 3 is a schematic side view explanatory of construction and operation of a conventional Cassegrain antenna;

FIGS. 4 and 5 are schematic side views explanatory of defects of conventional antennas;

FIG. 6 is a schematic side view illustrating an embodiment of this invention applied to a paraboloid antenna; and

FIG. 7 is a schematic side view illustrating and embodiment of this invention applied to a Cassegrain antenna.

To clarify the principles of this invention, conventional antennas and their disadvantages will be described first In a conventional paraboloid antenna shown in FIG. 1, a radio wave 3 radiated from a primary horn radiator l is reflected by a reflector 2 and then travels parallel to the axis of the reflector 2 as shown by a reference 3a. However, a part of the radio wave 3 is again fed back to the primary horn radiator l as shown by a reference 4. This is referred to as reaction by reflector, and this reaction affects the impedance matching characteristic of the antenna. This reaction is mainly caused by reflection at the periphery of the center of the reflector. According to the conventional devices, a small disk 5 for matching (i.e., a matching plate) is usually provided at this place as shown in FIG. 2, and the size of the plate 5 and a relative position of the plate with respect to the reflector 2 are adjusted so as to cancell out the above-mentioned reflected waves 4 by waves reflected by the small disk 5 at the feed radiator 1. However, this adjustment must be performed in a non-reflective room or in the open air to prevent reflection from other devices. Accordingly, this adjustment is troublesome, while a good matching in a wide frequency band is difficult.

In an example of a conventional Cassegrain antenna shown in FIG. 3, a spherical wave 3 radiated from a primary horn radiator l is reflected by a sub-reflector 8 towards a main reflector 2. The subreflector 8 is a hyperboloid having focuses F and F The main reflector 2 forms a paraboloid having a focus F so that a plane wave 3a reflected by the main reflector 2 travels along the axis of the main reflector 2. In this case, if a cone reflector is employed as the primary horn radiator 1 to directly radiate therefrom a plane wave, the surface of the sub-reflector 8 forms a paraboloid. These Cassegrain antennas are broadly used, as an antenna of large aperture, in the fields of radio astronomy and space communication since this Cassegrain antenna has a minimum length of feeder waveguide shorter than the paraboloid antenna and can be used in a wide frequency range.

However, if it is necessary to design this Cassegrain antenna by the use of a reflector having a smaller sperture (e.g.; 3 to 4 meters in diameter for 2 to 4 GHZ) so as to be used for a terrestrial radio repeating station, a ratio of a diameter of the primary horn radiator (and the sub-reflector) to the diameter of aperture of the main radiator becomes larger. Accordingly, aperture efficiency of the main reflector 2 decreases in accordance with increase of the diameter of the subreflector 8. In FIG. 4 showing this condition, a path 6 shows the radiation course of radio waves starting at the feed radiator l and reflected by the sub-reflector 8 and the main reflector 2 so as to radiate along the edge of the sub-reflector 8 in the direction of the axis of the main reflector 2. A path 7 is a radiation course of radio waves starting aim-radiator l and reflected by the sub-reflector 8 so as to again feed back to the feed radiator 1. Moreover, angles 0 and 0 are respectively an angle between the path 7 and the axis of the main reflector 2 and an angle between the path 6 and the axis the main reflector 2. As understood from this condition, all the waves radiated from the primary horn radiator 1 within the angle 0, are intercepted by the primary horn radiator 1 so that radiation to a desired direction cannot be performed. Moreover, all the waves radiated between the angles 0, and 0 are reflected by the sub-reflector 8 so that a part thereof are again fed back to the primary horn radiator 1. In other words, only radio waves reflected at a region a on the surface of the sub-reflector 8 are radiated as available waves to a desired direction. On the other hand, all the waves reflected at a region b on the surface of the sub-reflector 8 cause the above mentioned reaction, which affects an impedance characteristic of this antenna. Moreover, all the waves reflected at a region 0 on the surface of the sub-reflector 8 become unnecessary waves radiated to undesired directions and affect the radiation pattern of this antenna.

To avoid the above described affection of the impedance characteristic in conventional arts, a matching plate similar to the matching plate 5 shown in FIG. 2 is provided, or a projection 11 is provided at the center portion of the sub-reflector 8 as shown in FIG. 5 so that waves radiated from the primary horn radiator 1 within the path 7 are dispersed. (See Peter Foldes, et al.; Theoretical and Experimental Study of wideband Paraboloid Antenna with Central-reflector feed, RCA Review, Mar. 1960). However, satisfactory results have not been obtained.

The above mentioned problems are effectively resolved in accordance with this invention. With reference to FIG. 6, a wave absorber 9 is provided at the center portion of the main reflector 2 in accordance with a feature of this invention. Any of many kinds of wave absorbers, such as rubber-ferrite or ferrite-carbon-rubber, may be employed as the wave absorber 9. The diameter of the wave absorber 9 is designed so as to be equal to or slightly larger than the diameter of the matching plate 5. Moreover, the thickness of the wave absorber 9 is determined so as to be suitable for a desired frequency range and other usage conditions. The wave absorber 9 may be adhered by adhesives, such as alone-alpha. To raise the durability of the wave absorber 9, this wave absorber 9 may be protected by protective material having no transmission loss for radio waves, such as glass-fiber, provided on the surface of the absorber 9.

As the result of the above-mentioned construction, matching of the antenna which is troublesome in the conventional arts due to the above mentioned reaction can be readily performed, since disturbing feed-back of radio waves from the reflector 2 to the feed radiator 1 can be almost eliminated.

In another embodiment of this invention shown in FIG. 7, a wave absorber 10 is provided at the center portion of a sub-reflector 8 of a Cassegrain antenna. The size of this wave absorber 10 is designed so as to be smaller than the region c in FIG. 4 and larger than the region b in FIG. 4. Installation of this wave absorber 10 can be performed similarly to the manner mentioned with respect to the wave absorber 9.

As a result of the above construction, the reaction of radio waves caused by a sub-reflector in a case where a primary horn radiator and the sub-reflector each having a relatively large diameter in view of the diameter of a main reflector can be effectively eliminated, so that the impedance characteristic is extremely improved. Accordingly, a Cassegrain antenna having a small aperture can be readily realized while this is very difficult by conventional techniques. Moreover, this Cassegrain antenna can be readily made in comparison with conventional Cassegrain antennas of high price and of large weight, while characteristics better than those of conventional ones can be realized. Accordingly, the antenna of this invention can be broadly employed in the field of microwave or millimeter wave communication.

In an antenna using a deep main-reflector or using a metal shield plate, reaction of radio waves caused at the periphery of the main reflector or the shield plate cannot frequently be neglected. In this case, impedance matching by conventional techniques (i.e., a matching plate) must be performed in consideration of this reaction. However, other wave absorbers may be provided at the periphery of the main reflector or a part (or a whole part) of the metal shield plate in addition to the wave absorber 9 or 10, so that all the reactions can be effectively eliminated in accordance with this invention.

What we claim is:

l. A Cassegrain antenna system comprising a main reflector for electric energy waves, a sub-reflector for directing electric energy waves toward said main reflector, a primary horn radiator for directing electric energy waves toward said sub-reflector, and wave-absorging means consisting of an article of electric energy wave-absorbing material disposed only on a limited area of a central portion of said sub-reflector for absorbing only a portion of said electric energy waves directed toward said sub-reflector, which said portion of said waves consists of only those said waves which would otherwise reflect back to said primary horn radiator; said limited area being substantially equal to a projection of the geometrical shadow area of the subreflector on the sub-reflector caused by a light from a point at infinity to said main reflector. 

1. A Cassegrain antenna system comprising a main reflector for electric energy waves, a sub-reflector for directing electric energy waves toward said main reflector, a primary horn radiator for directing electric energy waves toward said sub-reflector, and wave-absorging means consisting of an article of electric energy wave-absorbing material disposed only on a limited area of a central portion of said sub-reflector for absorbing only a portion of said electric energy waves directed toward said subreflector, which said portion of said waves consists of only those said waves which would otherwise reflect back to said primary horn radiator; said limited area being substantially equal to a projection of the geometrical shadow area of the subreflector on the sub-reflector caused by a light from a point at infinity to said main reflector. 